The invention relates generally to the measurement of electrostatic potentials, but more specifically to an electrostatic voltmeter having a current source load working in a high voltage environment.
In the past, probes and sensing circuits have not been as accurate as they could have been. Prior systems employed passive load devices to measure electrostatic potential and were disadvantageous because of insufficient current to charge stray capacitances when a switching transistor was turned off. Another disadvantage is that a rather complex mirror system was required to measure the electrostatic potential.
A device constructed according to one embodiment of the present invention is capable of measuring electrostatic potentials with insignificant current flow. Generally, such a device includes a probe working in conjunction with a circuit that receives signals from the probe and produces an output signal. The output signal, indicative of an electrostatic potential, may be used to drive an indicator or to control a process as a function of the measured electrostatic potential. Thus, the present invention may be used in the printing arts and, more particularly, in an electrophotographic reproduction system to control a xerographic process. Electrostatic voltmeters, or ESVs as they are called, are particularly well suited for measuring the surface charge potential of a photoreceptor which, in turn, allows automated adjustment of machine characteristics to achieve a high quality reprographic output.
It is a general object of the present invention to provide an ESV and method thereof that generates an electrical signal representing an electrostatic potential of an object.
To achieve this and other objects, the present invention provides an ESV having a probe and a sensing circuit that provides an electrical signal proportional to a measured electrostatic potential. The circuit comprises a high voltage input section that is subjected to an electrostatic potential to be measured. The high voltage input section generates a first signal representative of the electrostatic potential. The circuit further includes a high voltage current source connected between the high voltage input section and a high voltage potential. A voltage level determining section is connected to a node between the high voltage input section and the high voltage current source to provide an electrical signal that is proportional to the electrostatic potential.
According to another aspect of the present invention, an apparatus that provides an electrical signal proportional to an electrostatic potential comprises a high voltage current source load, which is active rather than passive. A drain circuit of the high voltage current source load connects to sensor circuitry. A source circuit of the high voltage current source load connects to a high voltage potential. A gate circuit of the high voltage current source load connects to a suitable biasing circuit to set the drain current in a high voltage MOSFET. A voltage level-determining device connects to a node between the sensor circuitry and the source circuit for providing an electrical signal that is proportional to the electrostatic potential being measured.
According to yet another aspect of the present invention, an apparatus for providing an electrical signal proportional to an electrostatic potential comprises an active load connected to a sensor circuit. The active load includes a drain circuit connected to the sensor circuitry, a source circuit connected to a high voltage potential, and a gate circuit connected to a high voltage power supply regulator. A voltage level-determining device is connected to a node between the sensor circuit and the source circuit for providing an electrical signal that is proportional to an electrostatic potential.
In addition, the invention includes a method of measuring the electrostatic potential of an object. The method includes the steps of placing a probe at or in the vicinity of an object having an electrostatic potential on the surface thereof, and generating a first signal that is representative of the electrostatic potential. The method includes applying the first signal to an input of a sensor circuit for controlling the current flow within. The method further includes connecting an active load between the sensor circuit and a high voltage potential. Finally, the method uses a voltage level-determining device that is connected to a node between the sensor circuit and the active load to provide an electrical signal that is proportional to the electrostatic potential.
Other objects, features, and advantages of the present invention will become apparent when considered in conjunction with the accompanied drawings. The invention, though, is pointed out with particularity by the appended claims.